Stroke: custom arrowheads from an arrowheads file join the arrowhead menus

The Stroke panel menu's Edit Arrowheads… (stroke.arrowheads.edit) opens
Arrowheads.vectorcraft next to the preferences, creating it with one
example head when it is missing. Each symbol in the file is one head,
listed by name after the built-in heads in both arrowhead menus: it
points right, its registration point is the tip, and its height is drawn
as wide as the built-in heads. The file is read at startup and again
each time VectorCraft saves it; stroke.arrowheads.reload and
stroke.arrowheads.list read and list it.

Arrowhead gets a Custom variant, appended after the built-in names, that
carries the head's name and outline, so a document that uses one draws,
saves, reopens and exports it without the arrowheads file. stroke.set
takes {"custom": name} or the {"Custom": {name, path}} form that
document.inspect reports.

Part of #5421.

docs/mcp.md

@@ -3741,6 +3741,31 @@ the `…Open` rings…) and marks (`Bar`, `DoubleBar`, `DotOnBar`, `Slash`, `Dou
 `arrowScale`, at least 0.25 pt. `LineArrow` has two arms of the head's weight (the line's weight at 100 % for lines of
 0.25 pt or more) at right angles and spreads twice as wide.
 
+Custom arrowheads come from the arrowheads file, `Arrowheads.vectorcraft` next to the preferences (none on the web or
+in runs with `VECTORCRAFT_NO_PREFS`). `stroke.arrowheads.edit` (Stroke panel menu › Edit Arrowheads…) opens it, and
+first creates it with one example head when it is missing. Each symbol in the file is one head, named after the
+symbol and listed after the built-in heads in both arrowhead menus. These rules are VectorCraft's own: a head points
+right, the symbol's registration point is its tip, and the art's height is drawn as wide as the built-in heads (four
+head weights). The visible fills and the outlined visible strokes of its paths, compound paths and groups, and its
+gradient meshes, make the head, with geometry effects applied and clip groups clipped, and the line stops halfway along its length. A head is
+one solid shape in the stroke's paint: art at 0 % opacity is left out, and other opacities, blend modes and opacity
+masks do not change the shape. The file is read when the app starts and again each time VectorCraft saves it;
+`stroke.arrowheads.reload` reads it again. A file larger than 64 MB is not read, at most 256 symbols are read, and a
+symbol that paints nothing, reaches more than 64 head weights from its tip, has more than 65,536 anchors in its art or
+in the shapes it paints, or has more than 4,096 anchors once united is listed in `skipped`. `stroke.arrowheads.list` returns each head's `name`, `reach` and `halfWidth`, the `file`
+path, the `skipped` symbols and any read `error`.
+
+`stroke.set {endArrow: {"custom": name}}` puts a custom head on a stroke. The stroke keeps a copy of the head's art,
+so the document draws it, saves it and exports it to SVG and PDF the same way without the arrowheads file.
+`document.inspect` reports such a head as `{"Custom": {name, path}}`, which `stroke.set` takes back unchanged. The
+help page [Apply stroke on an object](https://helpx.adobe.com/illustrator/using/stroke-object.html) describes
+defining custom arrowheads in a separate arrowheads file.
+
+```json
+{"name":"run_command","arguments":{"command":"stroke.arrowheads.list","params":{}}}
+{"name":"run_command","arguments":{"command":"stroke.set","params":{"ids":[9],"endArrow":{"custom":"Example Arrowhead"}}}}
+```
+
 ## Stroke bounds and clicks
 
 Visual bounds (Fit to Selected Art, Rasterize, exporting all art) and clicks on the canvas take

docs/ui-parity.md

@@ -406,6 +406,7 @@ next unit in the order a right-click on a ruler lists them, from Feet back to Po
 | Select › Start Global Edit / Stop Global Edit; Properties › Quick Actions with the Global Edit Options popover (Match: Appearance, Size; On Artboards: All Artboards or a range, Include Objects On Canvas; Result(s)); Escape stops it; the matched objects are highlighted in their layer colour while it runs; a Shift-click on a highlighted match leaves it out | partial | #3521, `select.globalEdit` / `select.globalEditOptions` / `select.globalEditExclude`, `crates/engine/src/tests/globaledit.rs`; paths, compound paths, symbol instances and groups of them (similar groups, #4939: edits inside one group reach the same object in the others; [public source](https://helpx.adobe.com/illustrator/desktop/manage-objects/edit-objects/edit-groups-with-similar-objects.html)); Shift-click to leave a match out ([public source](https://helpx.adobe.com/ph_fil/illustrator/how-to/global-editing-sensei.html)); the Control bar's Global Edit Options dropdown opens the same options; rotated copies and highlighting matches before the start remain ([public source](https://helpx.adobe.com/illustrator/desktop/manage-objects/edit-objects/edit-similar-objects.html)) |
 | Contextual task bar | done | |
 | Control bar with Transform and Stroke popovers | done | |
+| Stroke panel: custom arrowheads after the built-in ones in both arrowhead menus, from the arrowheads file that the panel menu's Edit Arrowheads… opens; saving the file reloads the menus, and a stroke keeps a copy of its custom head's art | done | [Apply stroke on an object](https://helpx.adobe.com/illustrator/using/stroke-object.html) (Customize arrowheads); the file's layout (one symbol per head, pointing right, tip at the registration point, height drawn as wide as the built-in heads) is VectorCraft's own; `stroke.arrowheads.edit`, `stroke.arrowheads.list`, `stroke.arrowheads.reload`, `stroke.set` `{custom: name}`; engine `tests::arrowheads`, `stroke::tests_arrows::custom_heads_draw_their_own_art_at_the_tip` |
 | Control bar: Paragraph: link and Align left/center/right while type is selected (#5372) | done | After the Character controls, as the public documentation has the Control panel format paragraphs ([Paragraph panel overview](https://helpx.adobe.com/illustrator/using/formatting-paragraphs.html), [Align text](https://helpx.adobe.com/illustrator/using/align-text.html)); the link opens the Paragraph panel in a popover, as Character: does with the Character panel. The bar trims from its right end: X/Y/W/H, Distribute, Align, area type vertical alignment, then these buttons, then the link. The three-button set is our choice for room; all seven alignments are in the popover. `panels/paragraph/tests.rs` headless tests (`the_control_bar_shows_the_paragraph_alignment_of_the_selected_type`, `the_control_bar_trims_its_paragraph_section_after_the_controls_to_its_right`) |
 | Pathfinder shape results keep their source names and SVG IDs | done | #1293; Minus Front keeps the back name, Minus Back the front name; Unite/Intersect/Exclude use the backmost named input; compound output carries it once; geometry and paint unchanged |
 | Attributes fill-rule controls with a compound member selected | done | Edits the compound's rule; direction still edits only the member; rendered-hole and Undo/Redo regression tests (#1197) |